Those emails that pose as you are more then likely email worms and basically replicate itself either by using someones address book and then posing as all those people to send it to all those peoples addresses books and uses an smpt server to do it so you dont actually notice it sending or not sending…
I have also heard of ones that basically have a list of like say 50 names that are common. And it just takes each name on the list and then uses random persons addresses books, uses the domains from each email entry from the address book combines with the list of common names and sends them that way too…. So eventually your going to be looking like you sent it when you actually never did, nor did you get infected….its just the way it is lol